The present invention relates to a beam-hardening post-processing method for correcting the intensity of X-rays to be transmitted by a subject on the basis of phantom data, and to an X-ray computed tomography (CT) system.
An X-ray source employed in an X-ray CT system generates X-rays that fall within a certain energy range. On the other hand, an absorption coefficient for X-rays transmitted by a subject depends on the energy in the X-rays. The larger a length in a subject over which X-rays are transmitted, the larger the average energy. This phenomenon is referred to as a beam-hardening effect. Consequently, a proportional relationship is not established between the intensity of transmitted X-rays, that is, a projection information value and the length over which X-rays are transmitted, but a non-linear relationship is.
The beam-hardening effect causes the cupping effect signifying that the intensity detected in the center of a reconstructed image is low. A correction coefficient to be used to correct projection information values based on which a reconstructed image is produced to exhibit a uniform intensity is calculated in relation to each of the channels of the X-ray detector, whereby the correction is achieved (refer to, for example, Patent Document 1).
A plurality of cylindrical phantoms having different diameters that are large enough to generally cover the entire field of view (FOV) (scan field) defined in the center of an X-ray field is scanned for the purpose of higher-precision correction. Projection information acquired from the phantoms is used to improve the precision in correction.
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei 5(1993)-130987 (p.2 and 3, FIG. 1 and FIG. 2)
By the way, for highly precise correction of projection information values, the largest possible number of different projection information values is needed in relation to each of the channels of an X-ray detector. Therefore, lots of phantoms that have different diameters must be scanned.
On the other hand, the larger the diameter of a phantom, the lower a signal-to-noise ratio reflected in an acquired projection information value. If a correction coefficient calculated from projection information values reflecting lowered signal-to-noise ratios is used to correct projection information, a reconstruction image is like to suffer degradation in image quality deriving from a ring artifact or the like. Consequently, when a subject having large dimensions is scanned, image quality is likely to be degraded.